I guess you are new to Objectivism, and this is an excellent question. There is very definite evidence that theft is wrong, but it, like the evidence for the principle that oxygen supports combustion, cannot just be pointed to, saying, "see, that is the evidence." There is a chain of reason based on the evidence of the nature of the world we live in and the nature of man and the requirements of his nature.
The long answer is the whole Objectivist ethics, which is most succinctly explicated in Ayn Rand's The Virtue of Selfishness.
There is also an excellent complete online overview of Objectivist Philosophy here: Importance of Philosophy
But of course you want a short answer. But there are no short answers. Truth is neither simple to understand or easy to discover, and like all other things of value, requires effort and time to acquire. The desire for short easy answers to all questions is a kind of moral evasion of the requirements of reality.
Until you have time to pursue and discover the whole answer to you question, here is something from another publication of mine:
The question of normality in humans is complicated by the fact that humans are volitional beings. The behavior of all other creatures is determined directly by their nature. Human behavior is determined by their nature only to the extent that some behavior is required but no specific behavior is directly determined.
Does that mean man can do just anything? No, because man does have a specific nature, and whatever behavior is chosen must conform to that nature. For example, the unique psychological characteristic of human nature is volition, the ability and necessity to choose. Normal human behavior must conform to the requirements of this aspect of his nature:
A man may choose not to fulfill the requirements of his psychological nature, but he cannot live normally that way. It is not normal for a human being to live as a parasite, stealing or mooching from others who produce what his life requires, like a bloodsucker or louse. It is not normal for a human being to live as a pet or a slave of others, living on handouts from those he acts to please. It is not normal for a human being to live like a plant, depending on accident or luck, waiting for nature, fortune, or God to provide the things he needs to live and enjoy his life. These ways of living are normal for some organism, because it is their nature, it is not man's nature, however, and no man living contrary to his nature can live successfully or enjoy his life.
The immorality of stealing is based on the nature of, "property," which Objectivists regard as the product of human productive effort. A person's property is all they have produced by their own effort or acquired by trading the product of their efforts for the propety of others through mutual agreement.
Since the production of property requires both one's time and one's effort, stealing another's property is tantamount to taking away another's effort and time, that is, a portion of their life.
An Objectivist not only considers theft immoral, but regards any gain of value or property that is not earned or produced by one's own effort, theft. Most Objectivits, except under special conditions, do not like to receive gifts, because they so strongly and rightly feel, the unearned is immoral. Human being's must produce to live. That is an ultimate requirement of their nature. Anyone who limits the ability of another human being to produce or takes away what they have produced is interfering in another human being's pursuit of their own life.
That's the best I can do for a short answer for now.
Hank
A man may choose not to fulfill the requirements of his psychological nature, but he cannot live normally that way. It is not normal for a human being to live as a parasite, stealing or mooching from others who produce what his life requires, like a bloodsucker or louse. It is not normal for a human being to live as a pet or a slave of others, living on handouts from those he acts to please. It is not normal for a human being to live like a plant, depending on accident or luck, waiting for nature, fortune, or God to provide the things he needs to live and enjoy his life. These ways of living are normal for some organism, because it is their nature, it is not man's nature, however, and no man living contrary to his nature can live successfully or enjoy his life.
Any conclusion is easily reached when you start from a compatible premise a priori. Clear observable evidence not only does not support the contention above, but it directly contradicts it. Modern societies might be affluent enough to make the above true, but primitive tribal man depended dramatically on the group for survival. Individualism is a luxury of the comfortable (one reason we should be attempting to raise the standard of living for everyone). Humanity has always lived as parasites, whether from the land, the animals inhabiting it, or the other poor bastards who lived there first. Would this be true in a "perfect world"? Of course not. But you are the one who emphasizes observation, and history disproves your theory wholeheartedly. Show me the "normal" society based on the above dictums in history!
What makes America great is that it is decidedly NOT normal. It is a sparkling blip in human history; one of the few moments when human beings can live in freedom and prosperity. The idea that somehow freedom is "normal" smacks of the philosophies of the left, which declare that any change to the traditions and institutions of America can be made without destroying what makes America so unique in human history. Freedom is not "normal", which is why it is so precious!
Both your definition of "normal" and of "human nature" are incredibly arbitrary, and are based on philosophical theory, not "observation." All humanity might yearn to be free (in fact, most people don't... the pack instinct we inherited from our distant ancestors still manefests itself in myriad ways), but most are content to live within the bounds of society. Many will even reject freedom, as freedom is often less comfortable and less secure. That is why oppressive governments are so easily established and so long maintained, even by those who are oppressed. Your "observations", sir, are fantasy...
Your reply was, The immorality of stealing is based on the nature of, "property," which Objectivists regard as the product of human productive effort. A person's property is all they have produced by their own effort or acquired by trading the product of their efforts for the propety of others through mutual agreement.
Since the production of property requires both one's time and one's effort, stealing another's property is tantamount to taking away another's effort and time, that is, a portion of their life.
An Objectivist not only considers theft immoral, but regards any gain of value or property that is not earned or produced by one's own effort, theft. Most Objectivits, except under special conditions, do not like to receive gifts, because they so strongly and rightly feel, the unearned is immoral. Human being's must produce to live. That is an ultimate requirement of their nature. Anyone who limits the ability of another human being to produce or takes away what they have produced is interfering in another human being's pursuit of their own life.
That's the best I can do for a short answer for now.
OK. Your answer to my first question is, "Yes, there is clear objective reason from observable evidence that theft is wrong." But aside from this you just give a series of conclusions. Which leaves my second question, What, on your view, is the evidence?
I don't require a short answer. Just give me the first step or two of a long answer.